1. Field of the Invention
The present invention relates to an apparatus to detect a target in a radar, in particular relates to a signal processing apparatus in the radar which achieves the object by digital signal processing.
2. Description of the Prior Art
FIG. 1 is a block diagram showing signal processing in a signal processing apparatus in a conventional radar which performs the signal processing in a digital format, and the block diagram is disclosed in, for example, Radar Techniques, 5th ed. (The Institute of Electronics, Information and Communication Engineers), FIG. 3.11, at p.79.
In FIG. 1, reference numeral 1 means a band limiter, 2 means an A-D converter, 6A is a digital signal processor to perform, for example, MTI processing for reducing a reflected wave from fixed objects, and detecting only a moving target, 7 is a target detector, and 8 is a digital clock generator to feed a clock to digital circuits.
FIG. 2 is an explanatory view of an operation of the A-D converter 2, which is disclosed in FIG. 3.12 of the above reference. In FIG. 2, u and x.sub.i (i=1, 2, 3 . . . 7) are an input signal and an output signal of the A-D converter 2, respectively.
A description will now be given of the operation. In typical signal processing apparatus in the radar, since a narrow pulse is transmitted or pulse compression processing is performed so as to provide desired range resolution, a receive pulse has a sharp and narrow shape as shown by u in FIG. 2.
When the sharp and narrow receive pulse is quantized by the A-D converter 2 in the signal processing apparatus in the conventional radar, it is possible to provide a value shown by x.sub.i of FIG. 2. However, a time to perform the quantization is provided at intervals on a time base, and a relative position between a quantization point and a radar receive signal is not predetermined. Hence, it is not always possible to quantize the maximum value of the radar receive signal.
On the other hand, the target detector 7 has to detect the maximum value of the radar receive signal. However, in a characteristic of the conventional radar signal processing, the maximum value of the radar receive signal can not be quantized by the A-D converter 2, resulting in loss in signal detection.
There is an available technique to widen receive pulse to be quantized in order to easily quantize the maximum of the radar receive signal. However, it is impossible to provide the desired range resolution in the wide receive pulse. Hence, the A-D converter 2 is operated at a high speed so as to quantize an approximate value of the maximum value of the receive pulse for the purpose of reduction of the quantization loss. In this case, in order to correctly quantize the receive signal, the band limiter 1 limits a band of the receive signal within Nyquist frequency on the basis of the sampling theorem on the preceding stage of the A-D conversion. However, in view of implementation of devices, the high speed operation of the A-D converter 2 is limited so that the reduction of the quantization loss is also limited.
As set forth above, the signal processing apparatus in the conventional radar can not always quantize the maximum value of the radar receive signal. Thus, there is a problem in that the quantization loss occurs.
A signal processing circuit to perform conventional pulse doppler type MTI processing is disclosed in, for example, Radar Handbook, 2d ed. (McGraw Hill), FIG. 15.6 (at p.15.5) and p.15.34. FIG. 3 is a block diagram showing the pulse doppler type MTI circuit. In FIG. 3, component parts identical with or equivalent to those in FIG. 1 are designated by the same reference numerals, and descriptions thereof are omitted.
In FIG. 3, reference numeral 18 means a hit interval delayer to perform time delay for each pulse repetition period: T, 19A means a digital Fourier transducer, and 20A is a maximum value selector to select the maximum value in each output from the digital Fourier transducer 19A.
In the pulse doppler type MTI circuit, the A-D converter 2 performs A-D conversion of output from the band limiter 1. The hit interval delayer 18 performs the time delay to the resultant output obtained by the A-D conversion for each pulse repetition period: T. The digital Fourier transducer 19A performs the Fourier transform to each radar receive signal of each hit having the same range arranged in parallel to each other. Further, the maximum value selector 20A selects the maximum value in the respective outputs from the digital Fourier transducer 19A to feed the value to the digital signal processor 6A.
In this case, a characteristic of each filter bank of the digital Fourier transducer 19A is shown by V.sub.i in FIG. 4, and an output from the signal processing apparatus is shown by S.sub.v for example. However, there is another problem in that S.sub.v /V.sub.i to a target signal does not always become the maximum value in the pulse doppler type MTI circuit.